1. Field of the Invention
The present invention relates to a traveling-wire EDM (electrical discharge machining) apparatus and, more particularly, to improvements in a nozzle unit used for injecting a machining liquid to a workpiece.
2. Description of the Prior Art
FIG. 6 schematically shows the constitution of an exemplary traveling-wire EDM apparatus known heretofore. In this diagram, there are included a wire electrode 1; a wire bobbin 2 on which the wire electrode 1 is wound; an electromagnetic brake 3a; a brake roller 3 connected to the electromagnetic brake 3a and serving to apply a predetermined tension to the wire electrode 1; idlers 4a, 4b, 4c for changing the traveling direction of the wire electrode 1; a workpiece 5 to be machined; a machining liquid 6; an upper nozzle 7 for injecting and supplying the machining liquid 6 from above to a desired portion of the workpiece 5 being machined; an upper positioning guide 8 disposed in the upper nozzle 7 and serving to support the wire electrode 1; a lower nozzle 9 for injecting and supplying the machining liquid 6 from below to the desired portion of the workpiece 5 being machined; a lower positioning guide 10 disposed in the lower nozzle 9 and serving to support the wire electrode 1; a pump 11 for delivering the machining liquid 6 to both the upper and lower nozzles 7, 9; a machining power source 12 for applying a voltage between the wire electrode 1 and the workpiece 6 to generate electrical discharges; and a takeup roller 13 for winding and taking up the wire electrode 1.
Now an operation performed in the above apparatus will be described below. During an EDM operation, the wire electrode 1 is unwound and forwarded from the wire bobbin 2 and, with directional changes by the idlers 4a, 4b and 4c in the traveling path, a tension is applied by the brake roller 3 connected to the electromagnetic brake 3a, and finally the wire electrode 1 is wound on the takeup roller 13. With regard to the portion of the workpiece 5 to be machined, the wire electrode 1 is guided thereto while being supported by the upper and lower positioning guides 8, 10 in such a manner that the wire electrode 1 and the workpiece 5 are opposed to each other via a narrow gap. Then a voltage from the machining power source 12 is applied between the wire electrode 1 and the workpiece 5. When dielectric breakdown is caused across the narrow gap by such voltage applied, an electrical discharge is generated and the resultant thermal energy derived therefrom melts and removes a partial material of the workpiece 5. In such a state, the wire electrode 1 and the workpiece 5 are relatively displaced while an adequare narrow gap is maintained therebetween by an unshown controller and an unshown driving mechanism, and generation of electrical discharges is repeated for machining the workpiece 5 so that a desired contour is formed by the wire electrode 1. During the EDM operation, it is customary that the machining liquid 6 is delivered via a pump 11 to the upper and lower nozzles 7, 9 and is thereby injected toward the narrow gap for the purpose of ejecting the machining chips or detritus from the narrow gap and recovering the insulation therein while cooling the wire electrode 1 subjected to the high temperature due to the electrical discharges. The gap between the workpiece 5 and each of the upper and lower nozzles 7, 9 is normally set within a range of 0.1 mm to the order of 1 mm and, since the slit width of the contour to be machined is at most 0.5 mm or so, the machining liquid 6 injected toward the narrow gap is not delivered entirely thereto but is divided into a flow 10b supplied to the narrow gap and another flow 10a advanced along the surface of the workpiece 5, as illustrated in FIG. 7.
In the EDM operation, raising the machining speed is important as viewed also from enhancement of the machining efficiency. And such object can be achieved with facility by increasing either the discharge energy or the number of discharges. In such a case, however, the amount of machining chips or detritus is also increased with sharp temperature rise in the wire electrode 1, so that the EDM operation is rendered unstable with another drawback of inducing breakage of the wire electrode 1.
For eliminating such disadvantages, it is necessary to enhance the capabilities of ejecting the chips and cooling the wire electrode 1 by raising the flow speed and the flow rate of the machining liquid 6 injected from the upper and lower nozzles 7, 9. However, regardless of increasing the delivery pressure of the pump 11, the machining liquid 6 is caused to flow mostly along the surface of the workpiece 5 as denoted by 10a, and nearly none of the machining liquid 6 is supplied to the narrow gap where the liquid is essentially required, thereby posing another problem that the desired raise of the machining speed fails to be attained. With regard to solution of such problems, there are known some improved techniques as disclosed in, for example, Japanese Patent Laid-open Nos. 61 (1986)-61717, 61 (1986)152326 and Japanese Utility Model Laid-open No. 59 (1984)140134.
FIG. 8 illustrates the technique disclosed in the Japanese Patent Laid-open No. 61 (1986)-61717 mentioned, wherein sponge-like members 100 for preventing leakage of a machining liquid are so disposed as to surround the peripheries of upper and lower nozzles 7, 9 respectively, and one end of such member 100 is rendered slidable while being kept in contact with a workpiece 5, so that the machining liquid 6 injected from the upper and lower nozzles 7, 9 is supplied to a narrow gap without leakage to the outside. In FIG. 9 illustrating another known technique disclosed in the Japanese Patent Laid-open No. 61 (1986)-152326 mentioned, liquid screening rings 201 are provided slidably in a manner to be urged elastically by springs 200 toward a workpiece 5 and are thereby pressed against the outer surfaces of upper and lower nozzles 7, 9 respectively. Such liquid screening rings 201 are kept in sliding contact with the workpiece 5 and serve to supply, without leakage to the outside, the machining liquid 6 injected from the upper and lower nozzles 7, 9 to the narrow gap. And the further prior technique disclosed in the Japanese Utility Model Laid-open No. 59 (1984)-140134 mentioned is illustrated in FIG. 10, wherein an O-ring 300 is held at the fore end of a lower nozzle 9 in a manner to be kept in sliding contact with a workpiece 5, and such O-ring 300 serves to supply, without leakage to the outside, the machining liquid 6 injected from the lower nozzle 9 to the narrow gap.
As described above, both the flow speed and the flow rate of the machining liquid 6 delivered to the narrow gap can be increased to realize a higher machining speed.
However, there exist some disadvantages in the constitution of the conventional machining-liquid injection nozzle unit employed in the traveling-wire EDM apparatus. In the examples of FIGS. 8 and 10 where the sponge-like liquid leakage preventive member 10 or the O-ring 300 is used, a lateral force is exerted on the fore end of the upper or lower nozzle 7, 9 due to the sliding frictional resistance between the workpiece 5 and the leakage preventive member 100 or the O-ring 300, thereby causing some positional displacement of the upper or lower positioning guide 8, 10 incorporated in the leakage preventive member 100 or the O-ring 300 to consequently bring about deterioration of the machining precision. And further the leakage of the machining liquid 6 is increased in accordance with wear of such leakage preventive member 100 or the O-ring 300 induced by the sliding friction, hence causing reduction of the machining speed. Also in another prior example of FIG. 9 where the liquid screening ring 201 is kept in sliding contact with the workpiece 5 by the resilience of the spring 200, the spring 200 comes to be stretched as a result of increased wear of the liquid screening ring 201, and the pressure of the spring 201 is thereby decreased to consequently bring about leakage of the machining liquid 6, hence lowering the machining speed as in the other prior examples.